Shielded anode electron multiplier



June 4, 1940. P. "r. FARNSWORTH ET AL 2,203,043

SHIELDED ANODE ELECTRON MULTIPLIER Filed June 13, 1938 0 2 2 JWVVV .26 k. Z6- g 25 IN VEN TORS, PH/LO 7f FARNSWORTH A TTORNEYS.

Patented June 4, 1940 .smarioen mom EIECTRON mmuim Philo 'l. Farnsworth, Springfield Township,

Montgomery County, Pa., and Richard L Snyder, Glassboro, N. 1., assignors, by meme assignments, to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Del- Application'lune 13, 1938, Serial No. 213,334 a 4 Claims. .(Ci. 250-174) This invention relates to electronmultipliers of the D. C. type, and particularly to an electron multiplier having a shielded collecting anode.

Various types'of electron multipliers are now well-knownto the art, this application covering subject matter which is a further development of the general type of D. C. multiplier illustrated by the Philo T. Farnsworth reissue application Ser. No. 80,469, filed May 18., 1936, on an Electron multiplier, now matured into United States Patent Re. 20,759, granted June 14, 1938, and the Farnsworth and Snyder application Ser.

' No. 108,568, filed October 31, 1936, on a Repeater, now matured into United States Patent No. 2,143,146, granted November 1, 1939.-

The basic principle upon which all multipliers operate is that electron impact at sufficient velocity on a surface having a low work function will knock out a greater number of electrons, the ratio between secondaries and primaries being as high as ten to one under favorable conditions, and about six to one in normal operation. Successive impacts of a signal modulated electron current may be used to produce multiplication to any desired degree, limited only by the space charge effects. Further increases are obtainable by causing successive impacts to occur at different points on the secondarily emissive surfaces, in

which case the life of the secondary-electron' emitting cathodes is the ultimate limiting factor.

In the applications referred to above the electron drift required to produce successive impacts at points longitudinally displaced along continuous cathode surfaces is obtained by creating a potential gradient along an accelerating anode, and preferably also along the cathode surfaces, and placing a collecting electrode at the point of highest potential.

In the present case, means comprising a shielded anode construction are shown for using higher collection potentials on the anode than are customarily possible without drawing the electron cloud out of the multiplying space so rapidly as to interfere with the number of multiplying impacts desired. The objectsof this invention are: To produce a new type of D.'C. multiplier tube;

to produce a D. C. multiplier using an electron drift between successive multiplication-producing electron impacts; to provide an electron multiplier wherein the limiting current is dependent on the breakdown point of the secondarily emissive cathode surfaces; to provide a multiplier wherein the collective anode is partially shielded from the multiplying space; to provide a multiplier wherein a potential gradient is used along multiplying surfaces but wherein there is a unipotential space about a collecting anode; to provide a multiplier wherein high potentials may be applied to a collecting anode without limiting the number of multiplying impacts; and to provide a new and improved type of electron multiplier.

Other objects of this invention will be apparent or will be specifically pointed 'out in the description forming a part of this specification, but we do not limit ourselves to the embodiment of the invention herein described, as various forms may be adopted within the scope of the claims.

The functioning of this device may be better understood by reference to the figures, wherein:

Fig. 1 is a sectional view of the multiplier tube.

Fig. 2 is a circuit diagram illustrating a preferred mode of operation, omitting for clarity the focusing coil and electron gun shown in Fig. 1.

Referring now to Fig. 1, an elongated evacuated cylindrical envelope I is provided, having a neck 2 formed at right angles to an integral with the said envelope i. Neck 2.- carries a reentrant stem 4 terminated by,a press 5, through which are sealed leads 3 supporting a cathode, here shown as a filament 5, and a lead 1 which carries a centrally apertured control electrode 9. A gun type anode I0 is supported in proper alinement with the control electrode by a lead It sealed through the neck 2.

A high resistance, secondarily emissive surface I2 forms a snug fit within envelope i, extending substantially the full length thereof. An aperture is disposed through the surface opposite the electron gun anode III, in order that an electron beam may be directed into the space within the surface. A carborundum tube having a thin coating of metal sputtered on the inside has been found satisfactory for this use. The carbondum material resists the tube baking temperatures, possesses high resistance and considerable mechanical strength, can easily be turned to proper size, and affords a good surface to receive a coating such as thorium, beryllium, or other metal having a low work function which may later be oxidized.

Any coating having the properties of high resistance and high secondary emission is deemed a full equivalent of this embodiment, however.

A reentrant stem l5, sealed by a press It, is formed on the end of envelope l near stem 2, and through press it is fastened aneccelerating anode support ll. A reentrant stem l9 carrying a press 20 closes the opposite end of the envelope ,21 forming an accelerating anode, said anode being spot welded to frame 25, and to the accelerating anode supporting lead' H at the opposite end of the envelope.

Substantially enclosed within helix 26 'is a cylindrical collecting anode 29, fixed on collecting anode lead 2|. Near aperture 14, a lead 30 sealed through the envelope provides a low po- 9 tential end connection to the cathode surface l2, and at the opposite end, a high potential lead 3| is connected to the surface 12 and sealed through the tube wall.

A magnetic focusing coil 32 surrounds the tube along most of the multiplying chamber within cathode I2, and is energized by a D. C. source 34. In Fig. 2, the focusing coil 32 and source 34 have been omitted for the sake of clarity in representation, but it is to be remembered that these elements are retained in function.

In operation, a potential gradient is set up along cathode surface l2 by connecting the positive end of a voltage source 35 thereto through high potential lead 3|, and the negative end of said source 35 to the low potential end thereof through lead 30, which is grounded. A potential gradient is also set up along high resistance anode wire 21, by connecting the positive end of a D. C. source 36 to the lead 22, and the negative terminal of the source to anode lead l1. At the same time, the potential of the entire eccelerating anode and helix system is made positive with relation to cathode surface l2 by a connection 31 from the lead l1 and negativeend of source 36 to a tap 39 on source 3,5. This connection, for example, may raise the anode structure to about 100 volts above the cathode surface throughout the length of anode wire 21, assuming the potential gradients to be equal.

Collecting anode 29 is positively energized through a resistor 40 from a D. C. source 4|, the negative end of whichiis connected to lead 3| and the positive terminal of source 35, so that the collecting anode may be at a higher potential relative to ground than the accelerating anode and the cathode l2, depending on the relative value of sources 36 and II.

Since below saturation the output current is proportional to the initial number of electrons,

admitted to the multiplying chamber, an amplified output proportional to a signal current will be obtained if a signal is applied to control electrode 9 of the electron gun.

The path of the electrons through the tube is indicated by the arrows 42 in Fig. 2. Entering the aperture I! in the cathode l2, the electrons are attracted toward the accelerating anode wire 21, and are also displaced longitudinally of the anode wire by reason of the potential gradient thereon. At the same time, the magnetic field due to the focusing coil 32, causes the electrons to diverge laterally from a direct line to the anode 21, so that very few are collected thereon, the majority continuing across the tube to impact the cathode surface and produce secondaries. While the acceleration due to the anode 21 is followed by an equal deceleration as the electrons travel away from the anode, their displacement longitudinally of the tube subjects the electrons to a greater attracting potential on the cathode wall than existed-near aperture l4, and hence there is enough impact velocity to ensure ondary emission.

The secondaries again travel toward the acseccelerating, and collecting. anode structures; and

the multiplication proce repeats. For a full mathematical discussion of the effectof the focusing coil incausing the electrons to travel toward and past the accelerating anode without being collected thereon, see the Farnsworth reissue patent cited supra.

The number of multiplication producing trips is dependent on the potential gradients, and can be controlled by varying the potentials of sources 35 and 36. The helix 26 electrostatically shields the electrons near the h'ighpotential end of anode 21 somewhat from the charge on anode 29, creating a unipotential space thereabout, and hence permits the use of a high potential on anode 29 without withdrawing the electrons from the multiplying chamber before suflicient amplification is produced.

When the normal progression longitudinally of anode 29, and the potential drop created by the amplified signal flowing through output resistor isapplied to the desired output circuits through a blocking condenser 43 and a lead 42.

It will thus be apparent to those skilled in the art that there is provided an improved multiplier tube wherein close control may be exercised over the degree of amplification, and at the samev time, high anode potentials may be used.

Any conventional type of electron gun may be used, and various structural modifications may be made and different materials used without departing from the spirit of the invention.

The following is claimed:;

1. An electron multiplier, comprising an evacuated envelope having therein an elongated hollow high resistance and homogenous cathode structure internally capable of secondary electron emission, means for directing a modulated electron beam within said cathode, a high resistance accelerating -anode positioned centrally within said cathode, a collecting anode shield connected to said accelerating anode, a collecting anode disposed Within said shield, and a lead sealed through said envelope to each end of said cathode surface.

2. An electron multiplier, comprising an evacuated envelope, a high resistance cathode having a secondarily emissive cylindrical interior surface disposed within said envelope, said cylindrical surface bounding a multiplying chamber, a. reentrant stem carrying a press formed upon said envelope and extending axially toward one end of said mutiplying chamber, a U-shaped shielding anode-supporting and connecting frame member fixed in-and through said press, a collecting anode-supporting and connecting-lead sealed through said press, a cylindrical collecting anode fixed axially of said multiplying chamber to said collecting anode lead, a helical shielding anode fixed to said U-shaped member concen-- fixed to said U-shaped member and said accelerating anode supporting lead, connecting leads disposed through said envelope to each end of said cylindrical cathode surface, means for directing a signal-modulated electron beam into the end of said multiplying chamber opposite said collecting anode, and a focusing coil disposed about said chamber.

3. An electron multiplier having a continuous secondary electron emissive cathode surface defining a multiplying chamber, a linear accelerating anode of relatively small section disposed centrally thereof, a collecting anode disposed at one end of said accelerating anode and electrically distinct therefrom, means for establishing a magnetic field axially of said chamber, means for producing a stream of electrons directed at said emissive cathode surface, and shielding means for establishing a unipotential space about said collecting anode to permit said collecting anode to be operated at a higher potential than the potential of said accelerating anode.

4. An electron multiplier having a continuous secondary electron emissive cathode surface defining a multiplying chamber, a linear accelerating anode of relatively small sectiondisposed centrally thereof, a collecting anode disposed at one end of said accelerating anode and electrically distinct therefrom, means for establishing a magnetic field axially of said chamber, means for producing a stream of electrons directed at said emissive cathode surface, and means including a shield grid substantially enclosing said collecting anode for electrostatically shielding said collecting anode from said cathode surface.

PHILO T. FARNSWORTH. RICHARD L. SNYDER. 

